The present invention relates to digital compression systems for high definition television (HDTV) applications or the like, and more particularly to the communication of multiple channels of compressed video data via a multiplexed data stream.
Various different digital compression systems are known in the art for reducing the amount of data needed to adequately represent a sequence of video images. An example of such a system is provided in Paik, "DigiCipher - All Digital, Channel Compatible, HDTV Broadcast System," IEEE Transactions on Broadcasting, Vol. 36, No. 4, December 1990, incorporated herein by reference. In the system described in the aforementioned article, motion compensation is also provided to further enhance the image compression. The use of such systems makes it possible to achieve compression ratios in excess of 100 to 1. Most of the video compression algorithms used in these systems take advantage of statistical properties of the image. Occasionally, certain sequences of images will be encountered where these statistical properties do not apply. In such cases, a constant compression ratio cannot be maintained without visibly impairing the resulting image. In general, the variation in picture quality increases as compression systems become more powerful and more sophisticated. Usually, it is only the average compression ratio that is improved by such systems.
One method for reducing the variability of the statistical properties of video imagery is to simultaneously encode multiple video channels. If the quality of the reconstructed video signal is fixed, then the amount of data required to represent the signal after applying a particular compression algorithm can be described as a random variable. The total amount of data needed to represent n video channels is therefore the sum of n random variables. Assuming that each video channel carries different program material, then each of the video signals will be uncorrelated, and the variance of the sum of video signals will be much less than the variance observed by compressing a single channel independently.
It would be advantageous to provide a statistical multiplexer to process a plurality of video channels in a variable bit rate digital video compression system. The use of such a multiplexer would facilitate the achievement of a targeted compression ratio without introducing visible artifacts into reconstructed video images. Such a multiplexer should enable the transmission of multiple channel data without exceeding the throughput capability of a communication channel over which the data is communicated. The present invention provides a statistical multiplexer having the aforementioned advantages.